The present invention relates to an optical switch, for example, used in an optical communication system. More specifically, it relates to a waveguide type liquid-crystal optical switch for switching an optical path between cores by liquid crystal.
Optical communication has become popular in ordinary home use in recent years because a great volume of information can be transmitted/received at a high speed. The optical communication can be achieved by a transmission system using optical fibers. Various optical components such as a fiber type optical coupler, a waveguide type optical multiplexer/demultiplexer, a free-space propagation type optical multiplexer/demultiplexer, an optical switch, etc. have been developed to distribute an optical signal to respective terminals.
Of these optical components, the optical switch is important as an optical communication exchanger because it has a function of switching an optical path. Various types of optical switches are heretofore known as ones used in optical communication. Of these optical switches, an optical switch of the type using an optical waveguide for switching a propagating path of light by various kinds of physical phenomena has an advantage of high reliability and high speed because this type optical switch has no mechanically movable portion. As this type optical switch, there is known an optical switch using an optical waveguide such as a dielectric crystal waveguide of LiNbO3 having an electro-optical or acousto-optical effect, a semiconductor waveguide using carrier injection, or a silica waveguide using a thermo-optical effect.
An optical switch using liquid crystal is also known as this type optical switch having such an optical waveguide. Liquid crystal has an electro-optical effect in the wide sense in which the refractive index of the liquid crystal varies in accordance with application of electric field. Liquid crystal further has the following properties: it can be actuated by a low voltage; it has high reliability as represented by satisfactory results in use for display; and it can be produced efficiently and inexpensively. For example, such a waveguide type liquid-crystal optical switch has been described in JP-A-5-165068. The waveguide type liquid-crystal optical switch has a structure in which: two single mode optical core patterns having coupling portions parallel and close to each other are formed on a lower clad; a lower electrode is further formed on a part of the lower clad corresponding to the coupling portions; and the coupling portions are filled with oriented liquid crystal and sealed with a glass plate having an upper electrode.
In the waveguide type liquid-crystal optical switch, however, the liquid crystal and the lower electrode are formed so that the nearly whole of the lower clad is covered with the liquid crystal and the lower electrode. Accordingly, because the refractive index of the clad varies in a wide region of the clad to disturb the waveguide mode, large crosstalk regarded as being fatal to the switch is generated. Moreover, if there are some liquid crystal molecules not oriented in a predetermined direction, scattering loss due to the non-oriented liquid crystal molecules increases because the coupling portions of the waveguide cores come into contact with the liquid crystal at three surfaces. Further, loss due to the lower electrode becomes very large because the lower electrode is located extremely close to the coupling portions of the wavelength cores. In addition, there is a problem that polarization dependence is high because the optical switch has a structure in which no electrode but a pair of upper and lower electrodes can be disposed.